Final answer:
The opposing layers of the pleural membrane are held together by a thin film of serous fluid, which acts as a lubricant and reduces friction between the layers.
Step-by-step explanation:
The opposing layers of the pleural membrane are held together by a thin film of serous fluid. The pleura surrounding the lungs consists of two layers, known as the visceral and parietal pleurae. The serous fluid, which is secreted by the cells of the mesothelium, acts as a lubricant, reducing friction and abrasion between the two layers as the lungs expand and contract during breathing.